Restricted, repetitive, and stereotyped patterns of behavior comprise one of the core features used to diagnose autistic disorders. A wide variety of abnormal repetitive movements (dyskinesia, tics, stereotyped body and object movements) and abnormal repetitive behaviors (compulsions, rituals, insistence on sameness, restricted interests) occurs in persons with autism. Currently we know little about how these abnormal movements and behaviors are linked in the case of autism. Further, little is known about the pattern of repetitive behaviors that best discriminates autism from other developmental disorders. Preliminary findings suggest a distinction between "lower-order" repetitive behaviors (e.g. stereotyped body and object movements) and "higher-order" repetitive behaviors (e.g. rituals, sameness, restricted interests) in autism. The presence of a variety of simple repetitive behaviors that involve perseveration of motor actions suggests the involvement of motor control deficits. The presence of a variety of more complex repetitive behaviors that involve the rigid imposition of rules or the maintenance of a limited mental set suggests the involvement of specific deficits in cognitive flexibility. This model proposes that a combination of deficient motor control and cognitive flexibility processes occurs in autism that together constrains the ability to inhibit prepotent responding (stereotyped movements, compulsions), diminishes the ability to orient to novel events in the environment (insistence on sameness) and impairs the ability to generate flexible, adaptable patterns of behavior (rituals, restricted interests). To test the proposed model we will compare groups of children with autism (n = 90), children with nonspecific developmental disability (n = 60), and typically developing children (n = 90) on: (a) a standardized assessment (the Repetitive Behavior Scale) for identifying and measuring the variety of repetitive behaviors associated with autism (stereotypy, self-injury, compulsions, rituals/routines, sameness behavior, restricted interests); (b) a set of motor control tests (involuntary movement disorder exam, assessment of spontaneous eye blink rate, postural stability task), and (c) a set of cognitive flexibility tests (behavioral inhibition, set-shifting, novelty recognition memory, exploration task).